Much of the humor in Part I comes from the visual imagery of the contrast in size between Gulliver and the Lilliputians. The image of their hundred arrows shot into his hand that feel like the sting of needles seems funny because such little people can be so fierce and yet cannot do much to damage such a huge visitor. Other instances of humor revolve around Gulliver's physical needs, and again, most of these relate to the size difference. The Lilliputians feed Gulliver plates of a meat that he takes to be something like mutton legs, yet they look like tiny bird legs to him. When the little people transport Gulliver during his sleep to their city, he wakes up with a violent sneeze. Only weeks later does he learn that two inquisitive guards had climbed onto his face and stuck their spears up his nose.
Along with the humor about physical needs, the story contains scatological humor dealing with Gulliver relieving himself. The paragraph where he explains his first time moving his bowels after two days is funny because Gulliver makes it clear that he wishes to "justify my character." He feels ashamed that he defecated indoors in the temple because of he was too embarrassed to be seen in the act. Thereafter he went outside and did the necessary deed in the open air at the end of his chain, and before most Lilliputians were awake, some were assigned to cart the material away in wheelbarrows. More scatological humor occurs when Gulliver puts out a fire at the palace by urinating on the empress's chambers. This turns out to violate a law the kingdom has against urinating in the vicinity of the palace, but the emperor pardons Gulliver. The empress, however, is so horrified that she vows revenge and declares the rooms shall never be repaired.
Other humor in the section relates to the cultural oddities of the Lilliputians and satire about the cause of their war.
One of the funniest parts of Gulliver's adventures in Lilliput is when the Emperor orders him to stand, legs apart like a "Colossus," so that his army can march through Gulliver's legs. However, Gulliver says that, by this time, his pants were in such poor condition—full of tears and holes—that when the Lilliputians looked up, they were afforded a prime opportunity for "Laughter and Admiration." In other words, they could clearly see his private parts and, as Gulliver is twelve times their height, they were likewise impressed by the size of his parts.
Another part that I find quite amusing is when several of the Empress's rooms in the royal palace catch on fire and Gulliver, out of his desire to help, urinates all over the rooms to put out the flames. It seems not to occur to him how disgusting it would be to return to a room that has been completely soaked in urine. It would be better to just let the rooms burn! The Empress is so horrified that she refuses to return to the repaired rooms and vows revenge on Gulliver for this offense.